Grant-in-Aid for international Scientific Research
|Allocation Type||Single-year Grants|
|Research Institution||National Laboratory for High Energy Physics (KEK)|
TAKATA Koji National Laboratory for High Energy Physics (KEK), 加速器研究部, 教授 (10044760)
J.MADSEN CERN, PS, 主研
I. WILSON CERN, SL, 主研
W. SCHNELL CERN, CLIC, 主幹
K. HUBNER CERN, PS, 主幹
SHINTAKE Tsumoru National Laboratory for High Energy Physics (KEK), 加速器研究部, 助手 (90162792)
HIGO Toshiyasu National Laboratory for High Energy Physics (KEK), 加速器研究部, 助手 (10156581)
MIZUNO Hajime National Laboratory for High Energy Physics (KEK), 加速器研究部, 助教授 (80044775)
TAKEDA Seishi National Laboratory for High Energy Physics (KEK), 加速器研究部, 助教授 (40029898)
KIMURA Yoshitaka National Laboratory for High Energy Physics (KEK), 加速器研究部, 教授 (00010794)
MADSEN J. CERN
WILSON I. CERN
SCHNELL W. CERN
HUEBNER K. CERN
MADSEN J. CERN, PS, 主研
WILSON I. CERN, SL, 主研
SCHNELL W. CERN, CLIC, 主幹
HUBNER K. CERN, PS, 主幹
|Project Period (FY)
1991 – 1992
Completed(Fiscal Year 1992)
|Budget Amount *help
¥9,000,000 (Direct Cost : ¥9,000,000)
Fiscal Year 1992 : ¥5,000,000 (Direct Cost : ¥5,000,000)
Fiscal Year 1991 : ¥4,000,000 (Direct Cost : ¥4,000,000)
|Keywords||linear collider / linac / klystron / high gradient acceleration / high electric field / dark current / oxygen free copper / precision machining / リニアコライダ- / 加速管 / 高電界加速 / 無酸素銅ろう付 / 電界放出電子|
Experimental and theoretical studies have been carried out for accelerating structures of the main linac of linear colliders. The structure is of a traveling wave type driven with 11.4GHz X-band RF power. The accelerating gradient is desired to be as high as 100MV/m. In this fiscal year, we fabricated a proto-type structure of oxygen free copper with a very accurate and fine surface finish. It was then high power tested at KEK by using a 30MW klystron which had been developed at KEK. We were able to reach the high gradient after a 500 hour conditioning. We also made detailed measurements of dark currents due to surface emission under the very high RF electric field.
In order to achieve a high gradient, a high power klystron is absolutely necessary. However, the fabrication technology of klystrons becomes extremely difficult as the operation frequency raises. Therefore high gradient experiments were carried out at the S-band(2.86GHz), at which frequency the high power klystrons are easil
y available. In fact at this frequency we achieved over 80MV/m average gradients successfully and also tested beam acceleration too. For the TeV energy linear collider, however, the RF frequency should be an X-band(11.4GHz) one at the lowest, since otherwise the total ac power becomes impractically high. KEK has been developing X-band high power klystron for a long time, and recently we succeeded to get a first tube which can deliver 30MW RF power. This experiment is carried out by using this tube.
CERN has a different idea for the TeV linear collider. Particularly they are considering a K-band(30GHz) frequency generated by a FEL. Its realization is still far in the future. They are, on the other hand, developing technologies to fabricate accelerating structure which is capable of such high gradients. They are therefore wanting to test the structure as soon as possible even at lower frequencies. Hence a joint experiment started at KEK.
In this experiment CERN and KEK made structures of the same dimensions. The main specification is : number of cell 20, coupler cell 2, cell length 2.9mm, phase shift per cell 2pi/3, constant impedance structure, beam aperture 6mm, cell diameter 10.4mm, disk thickness 1mm. The Q value is 6500, attenuation is 1.54/m, impedance is 104MOMEGA/m. The maximum surface field over the beam aperture is 3.9 times the average gradient. The most critical point in the fabrication was to achieve a sufficient smoothness on the beam aperture. It is accomplished by carefully choosing diamond bits. We also paid attention to avoid metallic and organic dusts falling inside the structure. They are usually a serious cause to limit the maximum attainable gradient by inducing heavy discharges.
The test was carried out at the klystron repetition rate 50Hz. The RF pulse length was typically 100ns. The KEK structure reached 70MV/m after 60million pulses, While the CERN one did 100MV/m after 9 million pulses. In the CERN case, we also used a SLED system to boost the RF power.
Observed dark currents were analized by use of a modified Fowler-Nordheim formula. The important measure therein is the field enhancement factor beta. For ther both structures, we obtained similar values around 50 for beta.The absolute value of the dark current was found to be rather high for the KEK structure and further studies are necessitated to resolve this. Less